The unfriendly monster you have to find is called Don Tonberry. It's very rare and does only spawn in Bresha Ruins 100AF. You can only reach this location towards the end of the game (or after finishing the story). Head through the blue gate in 'A Dying World 700AF' (requires 1 wild artefact) to reach Bresha Ruins 100AF. Stay in the area where the atlas bossfight took place in Bresha Ruins 005AF. Run up and down the long path or keep running in circles. Don Tonberry spawns about once every 20 minutes (when you keep moving all the time). Capturing him is also based on luck! I had to defeat him 3 times before I captured his crystal (without fragment skills). You might need 10 tries if luck is not on your side. Remember that it's a black void that shows up when it's Don Tonberry.